b) opportunity costs i.e. the money I could have earned had I not spent the time writing my posts.

A is around 1000 euros a year.

B is harder to estimate. I would say 2000 euros a year.

Here is my reasoning: I probably spent at least 30 minutes per post writing and related activities like conferences, reading up and networking, so per week (assuming 6 posts) 3 hours. That's about 15 hours per month. So roughly 2 working days. So per year about 24 working days.

Now my opportunity cost is my day rate times 2, or 24/300 times year's salary. It now depends which rate you put in. I work as a consultant with a high day rate but few working days per year. So it is probably fairest to put in the salary of someone comparable that could run the blog. So I would go for the year's salary of a therapist or scientist. If I assume he gets net income of 2000 euros per month which is 24'000 per year multiplied by 0.08 (24/300), I get 1920 euros.

So I would say my replacement cost are 2000 euros to have a junior therapist /scientist doing it. But it would be much higher if I assumed I would work as a consultant instead.

So in total, I end up with 3000 Euros per year.

This would compensate my expenses, but of course I have high risk, no pensions, no healht insurance, job security, etc! :-)

Sounds good, I think most people would be willing to pack their lunch or eat at home (vs. eating out) to keep your excellent blog going. So don't be afraid to ask for money. There are some wealthy pws/readers out there...you can argue whether money should go to BSA or NSA or to your blog...

20 euros x 100 donations is 2000 euros. Sounds reasonable.

Of course, if you are getting paid, then you have to be professional and be responsible for the quality. It is just a blog, but a good blog. A blog can have big big impact nowadays.